What is DCS? (Distributed Control System)

In this video and article, we are going to do a deep dive into the DCS and discuss some of the why’s and how’s associated with the DCS. Let’s first clarify for the purpose of this lesson, what we mean by DCS.

What is a Distributed Control System? Over the years, the term DCS has evolved from the original description for the acronym as a “Distributed Control System” to the use of the term “Decentralized Control System” and they seem to be somewhat interchangeable nowadays.

Regardless of which description is used, we are discussing a structure that, at the high-level view, is a system that coordinates and supervises an entire plant of many varying processes.

An Overview of DCS and PLC Systems

Briefly, as a point of the historical review, when PLCs were invented, they were really good at handling single processes and were primarily used for repetitive, discrete control.

The advent of the DCS was for controlling many autonomous controllers that handled many continuous operations, mainly using analog control.

Through time and innovation, the lines have blurred a bit between the two systems but each, in the current day, has some principle differences.

PLCs, traditionally, were used for single batch or high-speed control, have a relatively simple, low-cost design, and are the core of the system. Their design is flexible and generic but completely customizable.

Processing time for tasks are typically very fast, operators usually interact and control the system using some sort of graphical display such as SCADA.

A DCS is used for continuous, complex controls, have an integrated control center much like a SCADA, which is the core of the system versus the processors in a PLC system.

The DCS has a number of predefined functions that come ready to customize and deploy for various applications.

Processing times are somewhat slower. Operators interact with the control system via an integrated graphical display.

DCS also have a claim that when safety is a top priority, it is the most reliable system.

The reason for this is because the manufacturer supplies both the control and supervisory equipment as an integrated package, the risks of integration errors are greatly reduced.

There are indeed scenarios in which a PLC system would be the best option such as smaller sized processes where you could employ redundant components to negate the possibility of process shutdowns.

Without redundancy, you risk production halts due to the nature of a single processor controlling an entire plant.

Redundancy may be deployed in either the PLC or DCS applications. We will talk more about redundancy in a future article.

Just as there are circumstances for a PLC system, the use of DCS would be for larger, more complex processes that require a lot of interaction between many processors.

Now that we’ve touched on some of the differences in the systems, let’s focus on the DCS and some of its components.

The DCS is a process-oriented system that uses closed-loop control.

Basic Components of a Typical DCS Network

1. Operator Stations

A typical plant starts with a centralized operator control center typically called Operator Stations.

Operator Stations, in a DCS, are the heart of the system.

This is where the operator can observe the operations of the plant, view process warnings and alarms, monitor production, and more.

2. Servers, Archiving and Engineering Computers

The next level of components may contain servers, archiving computers, and engineering stations.

Communications with the Operator Station level is typically Industrial Ethernet.

Servers are used for the collection of data at the processor level.

They are responsible for the data that moves between the Operator Station and the processors on the plant floor.

Archiving computers are used for storing historical data that may be used for trends or compliance.

Engineering stations are used for creating the projects on which the processes run. This includes;

– Hardware configurations

– Logic for tasks

– Graphical displays for operator interaction

– Administration of all of those tasks through installed software packages.

This is the station that is used to download the projects to the processors and the graphical displays.

3. DCS Controllers

At the next level, you have the master controllers that supervise the individual processors as well as I/O modules.

These controllers are also responsible for providing the data to the servers, which in turn, supply the data for the graphical interface.

Industrial Ethernet is typically used for communication with the previous level. Fiber Optic may be used here when Ethernet cabling runs would be too long.

At this level, the processor executes the logic and does what it needs to do in order to control the process.

Industrial Ethernet is typically used for communication with the previous level. Fiber Optic may be used here when Ethernet cabling runs would be too long.

At this level, the processor executes the logic and does what it needs to do in order to control the process.

4. Field Devices

The next level is the field device level. Communications between this level and the processor level can be nearly any type that may be compatible with the components.

Those include Industrial Ethernet, Profibus DP, EtherCAT, Fiber Optic, or other proprietary communication protocols.

Components at this level would be devices such as transmitters, switches, valves, motors, remote or distributed I/O, etc.

Summary

In short, both PLCs and a DCS have their place in the market today.

PLCs would work best in a small production environment where component failure if no redundant system is deployed, is a small risk for the application, the budget is restricted, or the tasks and I/O count are minimal.

A DCS would be better used in an environment where there are large I/O counts with many continuous processes, a processor failure in one section of the plant is not a problem for production, or risk assessment has determined that an integrated package would be the best option.

The line between the two systems is nearly invisible nowadays and it may not be long before there is no differentiation at all.

In future articales, we are going to discuss SCADA and contrast SCADA and DCS control systems. So stay tuned!

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The RealPars Team

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